This invention relates to an apparatus for the mixing of streams of fluids, including liquids and gases, insertable in a pipe of any cross section in which stationary mixing elements are used.
Mixing of two or more different substances is useful in many industrial applications. The substances may be any combination of solids, liquids and/or gasses. The substances may be miscible where mixing produces a single phase blend or immiscible, yielding a dual phase emulsion. A liquidxe2x80x94liquid emulsion is a dispersion of one liquid phase in another substantially immiscible continuous liquid phase. A gas-liquid dispersion is a dispersion of an insoluble or partially soluble gas into a liquid.
The art has typically used dynamic mixers employing axially rotating elements for the production of emulsions. By their very nature rotating elements such bars, pins, paddles, and the like do not have a uniform tangential speed. Consequently, when a fluid, flowing in the axial direction, encounters an element rotating an angle to the axis, typically perpendicular thereto, more shear will be imparted at the outer radius of the rotating element than at the center of rotation. This difference in applied shear makes preparation of uniform emulsions difficult because more than optimal shear may be imparted at the outer radius while less than optimal shear may be imparted near the center of rotation. Further, the differences in applied shear have different effects on the resulting emulsion, depending on the size of the rotating element. Such differences make scale-up difficult. Further, dynamic mixers require significantly greater energy input than static mixers, potentially jeopardizing the economics of operation.
For production of gas-liquid dispersions, liquidxe2x80x94liquid emulsions, and other mixtures the art has typically used static mixers to provide the shear and elongation necessary to disperse the discrete phase throughout the continuous phase. See, for example, U.S. Pat. No. 3,918,688 issued to Huber et al. on Nov. 11, 1975 incorporated herein by reference and U.S. Pat. No. 5,971,603 issued to Davis. et al. on Oct. 26, 1999, respectively. U.S. Pat. No. 4,019,719, issued to Schuster et al. on Apr. 26, 1977, and U.S. Pat. No. 4,062,524 issued to Brauner et al. on Dec. 13, 1977, both incorporated herein by reference, respectively describe an apparatus for thoroughly mixing components of fluid material through a tube-like conduit which contains a plurality of consecutive mixing elements comprising a set of stationary, angularly disposed flow deflecting baffles and an apparatus having a pipe with pairs of comb like plates arranged so that webs of one plate extend cross wise to the slots of the other.
In static mixers fluid flows past fixed elements is divided, stretched, folded and recombined by an arrangement of elements to provide mixing of all the substances present. A bar is an individual member which divides the flow. An element is an arrangement of bars, typically held mutually parallel, at any cross section in the flow path. Typically a static mixer may have from five to 30 elements, with as few as two elements being used for turbulent flow applications.
Prior art static mixers have also used steel wool for the internal elements, instead of the discrete bars described above. Steel wool has no fixed geometry. Variations in the density of the steel wool cause similar variations in the precision of the process in which such a static mixer is used. Further, portions of the steel wool may break off and be washed downstream. Prior art static mixers have also used corrugated sheets for the internal elements, instead of the discrete bars described above. Corrugated sheets have not been found to yield the tight particle size distribution sought by the end users of static mixers. Prior art static mixers have also used superimposed mesh screens, instead of the discrete bars described above. Mesh screens must be woven, increasing the fabrication cost and have the disadvantage of weak internals that may break, contaminating the process.
Frequently a commercial scale static mixer is derived from a bench scale static mixer which has proven suitable. Scale up for static mixers has attempted to hold constant shear rate and residence time in laminar flow applications and power per unit volume in turbulent flow applications. Thus, scale up from bench scale to commercial scale was usually done by holding the number of stages and bars constant while increasing the cross sectional area of the pipe or other flow channel.
In lieu of scale up, the art has utilized parallel processing to mix streams of fluids with multiple small mixers physically grouped together in order to increase scale of production, such that comparable product quality is achieved at various scales. Such xe2x80x9cgroupingxe2x80x9d designs pose difficulties for process control and reliability. For example, proper dosage of individual streams into each individual parallel mixer conduit is difficult to achieve. Moreover, the use of parallel systems (on the order of hundreds for large commercial scales) is impractical and costly.
Improvements in the method of reliably producing such mixtures, dispersions, and emulsions at a range of scales are needed. It is difficult to predictably scale mixers from a laboratory scale or pilot scale to a full production scale. Simply increasing the size of a static mixer to increase production capability (even if some process parameters, such as shear rate are matched) does not necessarily result in an dispersion/emulsion having the same properties as produced using a smaller scale static mixer.
In accordance with a first aspect of the present invention, a method for and static mixer for mixing two or more miscible or immiscible substances is provided. The method comprises the steps of providing a first phase and a second phase the ratio of said first phase to said second phase being between about 1:1000 and about 250:1; combining the first and second substances to provide a mixed process stream; using at least one static mixer in a single pass so as to provide sufficient surface area and residence time to mix the substances. In another aspect of the invention, a pilot or laboratory size static mixer is scaled to commercial size while holding constant the ratio of active surface area to void volume.